Go off to help a large retreating combatant (9)
I believe the answer is:
gladiator
'combatant' is the definition.
(I know that gladiator is a type of pugilist)
'go off to help a large retreating' is the wordplay.
'go off' becomes 'rot' (as in food going off).
'to help' becomes 'aid' ('aid' can be a synonym of 'help').
'large' becomes 'lg' (abbreviation).
'retreating' says the letters should be written in reverse.
'rot'+'aid'+'a'+'lg'='rotaidalg'
'rotaidalg' backwards is 'GLADIATOR'.
(Other definitions for gladiator that I've seen before include "One scrapped" , "one entering the ring" , "Man appearing in ring" , "Warrior" , "Film" .)